The present invention relates to data communication and more particularly to data communication over substantially orthogonal frequency channels.
Orthogonal frequency division multiplexing (OFDM) systems offer significant advantages in many real-world communication systems, particularly in environments where multipath effects impair performance. OFDM divides the available spectrum within a channel into narrow subchannels. In a given so-called “burst”, each subchannel transmits one data symbol. Each subchannel, therefore operates at a very low data rate compared to the channel as a whole. To achieve transmission in orthogonal subchannels, a burst of frequency domain symbols are converted to the time domain by an IFFT procedure. To assure that orthogonality is maintained in dispersive channels, a cyclic prefix is added to the resulting time domain sequence. The cyclic prefix is a duplicate of the last portion of the time domain sequence that is appended to its beginning. To assure orthogonality, the cyclic prefix should be as long as the duration of the impulse response of the channel.
To maximize the performance of an OFDM system, it is desirable that the response of the channel be known at the receiver end of the link. To provide the receiver with knowledge of the channel response, the transmitter typically includes training symbols as part of the frequency domain sequence. The training symbols have known values when transmitted and their values as received indicate the channel response. The number of training symbols should generally be greater than the length of the duration of the impulse response of the channel.
The use of training symbols or transmission of channel response information as data takes away from the data carrying capacity of the link. Furthermore, the number of symbols used for training in a given burst does not decrease when smaller bursts must be used, e.g., to reduce latency for voice traffic or decrease sensitivity to phase noise. For systems that employ short bursts, the efficiency loss due to training is even greater.
The discussion up until now has assumed a point to point link. However, the loss of data carrying capacity due to channel training is greatly compounded in point to multi-point systems where channel capacity is shared among many nodes. In a point to multipoint system, the channel response is different for every combination of access point and remote station. Each separate channel response must be learned, representing a great loss of efficiency.
One way of using OFDM in dispersive channels without the use of channel training is to apply differential coding or modulation to the frequency domain symbols. Such a differential scheme encodes data as phase differences between frequency domain symbols. Channel magnitude response thus does not corrupt data transmission because the receiver system does not take received magnitude into account in estimating the transmitted data. Phase magnitude response also does not corrupt data transmission because any phase difference applied by the channel is effectively subtracted out as a part of the differential decoding process.
Another useful communication technique is the use of multiple reception antennas. The resulting spatial diversity may be exploited to ameliorate the effects of interference. IMPROVED SYSTEM FOR INTERFERENCE CANCELLATION, U.S. application Ser. No. 09/234,629, filed on Jan. 21, 1999, the contents of which are herein incorporated by reference, discloses the application of spatial diversity to an OFDM system to ameliorate interference. The techniques disclosed there are heavily reliant on knowledge of channel characteristics. It would be highly desirable to optimally combine input from multiple antennas without knowledge of channel characteristics.